1. Field of the Invention
The present invention relates to a process for the preparation of 4-amino-2,2,6,6-tetramethylpiperidine (TAD) from 2,2,6,6-tetramethylpiperidine-4-one (triacetonamine, TAA), ammonia and hydrogen in the presence of a catalyst.
2. Discussion of the Background
4-Amino-2,2,6,6-tetramethylpiperidine is referred to as sterically hindered on account of the 2,2,6,6-substitution and can be used widely, in particular as intermediate in the production of UV stabilizers for polymers. It is important here that besides having a high chemical purity, the 4-amino-2,2,6,6-tetramethylpiperidine has no or the lowest possible intrinsic color. Even over a prolonged storage period of several months, no discoloration of the 4-amino-2,2,6,6-tetramethylpiperidine should take place. This is particularly critical when the 4-amino-2,2,6,6-tetramethylpiperidine is used for producing stabilizers or directly as additive since the product quality of the 4-amino-2,2,6,6-tetramethylpiperidine has a decisive influence on the quality of the stabilized polymers.
4-Amino-2,2,6,6-tetramethylpiperidine is generally prepared by a reductive amination of 2,2,6,6-tetramethylpiperidine-4-one in one or two stages in the presence of catalysts.
Thus, EP 0 776 887 A1 describes a continuous process at pressures of from 285 to 300 bar in the presence of metal catalysts selected from cobalt, nickel, ruthenium, palladium and platinum, and in the absence of solvents. The process according to DE 30 03 843 A1 is likewise carried out at pressures above 200 bar and without solvents and can also be carried out as a discontinuous process.
However, discontinuous processes are generally carried out at a lower pressure and in the presence of catalysts which have cobalt or nickel. Thus, EP 0 714 890 A2 describes a process at a pressure of 95 bar and likewise without solvents. Here, the 4-amino-2,2,6,6-tetramethylpiperidine is obtained in a purity of from 95 to 97%.
Often, however, processes are described which are carried out in the presence of solvents, such as, for example, water or alcohols. Thus, GB 2 176 473 and CN 1358713 A describe a process using water as solvent and in the presence of alkali metal and alkaline earth metals as cocatalyst at a pressure of from 10 to 30 bar. The yields attained are in the case of GB 2 176 473 90 to 95%, and in the case of CN 1358713 A 96.6%.
Li Yang et al. in Chemical Industry and Engineering Vol. 23 No. 4, 323-327 describe the influence of a number of process parameters on the process for the preparation of 4-amino-2,2,6,6-tetramethylpiperidine. Thus, Li Yang et al. describe that the selectivity can be increased through the use of cobalt catalysts instead of nickel catalysts. The suitable temperature is 90 to 100° C. and the hydrogen pressure should be 15 to 25 bar. As is also the case in the background art described above, options to improve the color stability of the 4-amino-2,2,6,6-tetramethylpiperidine are not described.
By contrast, WO 99/16749 describes a process for the purification of 2,2,6,6-tetrasubstituted 4-aminopiperidines in order to improve the color stability of these compounds. After the distillation of the 2,2,6,6-tetrasubstituted 4-aminopiperidine, this is reacted with hydrogen in the presence of a hydrogenation or dehydrogenation catalyst and separated off from the reaction mixture. Using this purification step, the APHA color number can be lowered to less than 10.
WO 97/46529 also describes a process for the purification of these piperidines. In this process, water and high-boiling compounds are firstly removed from the reaction mixture by distillation, a reducing agent is added and, finally, the piperidine is isolated by distillation. By adding a reducing agent, in particular NaBH4, the APHA color number can be lowered to less than 15.
The two PCT publications mentioned in the previous paragraphs each describe an additional purification step in order to achieve the desired color stability.